A large accumulation of data has implicated estrogen in breast cancer (BC) development. The catechol estrogen pathway has been a focus of BC research because particular catechol estrogen metabolites are genotoxic. That is, these estrogen metabolites are capable of forming DNA adducts which may lead to DNA mutations in a manner similar to benzene, a known carcinogen. The GER assay provides information on the proportion of estrogen metabolites that are bound to DNA base pairs in the urine in relation to unbound estrogen metabolites, thereby quantifying the extent to which estrogen metabolites have removed base pairs from the DNA. Our laboratory colleagues observed in 2 small-scale studies that GER was higher in BC patients and healthy women at high risk of BC compared to low-risk, healthy controls. While these studies support the hypothesis that BC development is influenced by GER, these data require replication in a large, prospective study. Methods: We propose to conduct a nested case-control study within the WHI cohort using urine samples collected prior to diagnosis from 360 BC cases and 360 matched controls in order to examine the risk of BC associated with GER (Specific Aim 1) and investigate whether dietary intake is associated with GER (Specific Aim 2). Our secondary aims involve exploring the presence of effect modifiers in the pathway involving GER, dietary factors, and BC (Secondary Aim 1) and examine whether a reduced set of estrogen metabolites are equally predictive of BC risk as the current 38 metabolites comprising the GER (Secondary Aim 2). Summary: The proposed study would be the first study using prospectively-collected samples examining the risk of BC in relation to this novel, estrogen biomarker. Leveraging the existing data from a top-rate study, such as WHI, provides an efficient method for addressing the research gaps surrounding GER. Our experienced, multi-disciplinary research team is well suited to carry out this important research project. Data emanating from this proposal could inform on the utility of the GER biomarker in relation to breast cancer.